The present invention relates to a process and an apparatus for developing radiation-sensitive, exposed printing forms with constant developer efficacy in a developing solution, the electric conductivity of which is continuously measured.
EP 107,454 discloses a processing process and apparatus for a number of imagewise-exposed radiation-sensitive plates which are brought into contact with a processing fluid. Changes in the electric conductivity of the processing fluid during the process are observed and the processing conditions are varied in response to changes in the electric conductivity. The processing fluid is either a developing fluid, a finisher or a hydrophilizing agent. Furthermore, a lithographically inert material which is ionized in solution may be added to the processing fluid, in order to increase the electric conductivity.
This apparatus for processing the imagewise-exposed, radiation-sensitive plates comprises a container for the processing fluid and a device for moving the plates along a path through the apparatus, so that they are in contact with the processing fluid. The apparatus includes a device for measuring the electric conductivity of the processing fluid and for generating an output signal in response to the conductivity and a device for changing the processing conditions in response to the output signal. The device for changing the processing conditions includes, inter alia, a variable speed motor for driving the plate-moving device, the motor being controlled by the output signal in such a way that the time interval during which the plates are in contact with the processing fluid depends on the conductivity of the processing fluid. In the case of this process and the apparatus provided for it, it is assumed that, with constant electric conductivity or constant conductance, an identical reproduction is always obtained after the development of the plates.
DE 3,007,401 discloses a process for developing an imagewise-exposed, positive-working, light-sensitive lithographic printing plate and for keeping constant the activity of an alkaline developer during the development. A first addition with a higher alkalinity than that of the alkaline developer is added upon each treatment of the light-sensitive material. A second addition with a higher alkalinity than that of the developer is added either continuously in a certain quantity or at certain intervals in certain quantities. The first addition or the first replenisher is added to the developer in a certain quantity proportional to the length of a side of the light-sensitive material to be treated.
The radiation-sensitive printing forms, in particular lithographic printing forms, to be developed into printing plates in the prior art, generally comprise a metal substrate, in particular an aluminum substrate, which is mechanically or chemically roughened in order to obtain a correspondingly suitable hydrophilic surface, to which a radiation-sensitive coating is then applied. Such printing forms with radiation-sensitive layers are exposed in an imagewise manner through a positive or negative transparency. The incident radiation changes the solubility of the radiation-sensitive coating. Subsequently, the imagewise-exposed printing form is processed by bringing the exposed printing form into contact with a developing solution in order to remove selectively those areas of the coating not required for the image development. After the developing step, the printing form is generally cleaned and washed and treated with a finisher and hydrophilizing agent, the main purpose of this step being to protect the developed printing plate and/or hydrophilize the non-image areas.
In the development of exposed printing forms into printing plates, the problem arises that the developer or the developing solution is weakened by the developing operation. This weakening of the developer leads to a darker or fuller image reproduction on the printing form, which in no way meets the requirement for a standardized printing of the finished printing plate.
The monitoring of the image reproduction is generally carried out by simultaneously copying test elements. As soon as a weakening of the developer efficacy is established by means of the test elements, the weakened developer solutions are refreshed or strengthened by additions of a replenisher. The efficacy of this replenisher addition is likewise tested by simultaneously copying test elements and/or by measuring the electric conductivity or the conductance.
In practice it has been discovered that different known developing systems comprising developer or developing solution and replenisher for refreshing the developing solution and for setting the developer efficacy to a constant value have a different dependence on the conductance or the electric conductivity of the developer solution over a certain period of time, the conductance being dependent on the throughput in terms of surface area of printing forms through the developing solution. For instance, in processing installations in which the development is performed by means of the conductance or by means of controlling the conductance, it is shown that, when changing over an already installed developing system to a new developing system, the previously practiced maintaining or constant control of the conductance, with the result of a constant developer efficacy, in the new developing system leads to a constant weakening or a constant increase of the developer efficacy, with the developing solution consequently becoming unusable. As a result, use of the ready-installed processing installation for the new developing system comprising developer and replenisher is not possible.